The evolution of galaxies in radio-selected groups

Abstract

The evolution of galaxies in small groups is studied in a way unbiased by selection effects, on the basis of observations of the populations of galaxies around a sample of 98 strong radio galaxies, 65 with redshifts less than 0.25, and 33 with redshifts between 0.25 and 0.50. The properties of galaxies with M(V) not greater than -19.0 within 0.5 Mpc of the radio galaxies are analyzed. Strong radio galaxies are found to occur in groups whose range of richness is broad, but not as broad as that of groups in general. There has been a slight decrease in the richness of radio groups since the epoch observed at z of 0.4, particularly among the FR II sources. The luminosity function of the members of both high- and low-redshift radio groups is indistinguishable from that of nearby field galaxies. It is concluded that galaxies in groups containing radio galaxies are typical of group members in general and are a good probe of the properties of normal galaxies. It is suggested that the morphology-environment relation is of recent origin and is steepening rapidly with time.